Since it is known that the metabolism of acetaminophen is involved in its hepatotoxicity and that drug metabolizing enzyme activity is decreased in tumor bearing animals, it was of interest to study the influence of L-1210 leukaemia on acetaminophen hepatotoxicity in BDF-1 male mice. A single oral dose of acetaminophen, 125 mg/kg, was given at the fifth day of the mice survival period (7.7 days) and the animals killed twenty-four hours later. As revealed by serum glutamic-pyruvic transaminase, serum glutamic-oxaloacetic transaminase and lactic dehydrogenase, acetaminophen was less hepatotoxic in leukaemic mice than in control mice by comparison with their own saline group; on the other hand the difference between control and leukaemic mice treated with acetaminophen was significant only for glutamic-pyruvic transaminase. Moreover, we found higher unchanged acetaminophen concentrations in plasma, liver, kidneys, brain and fat of the leukaemic mice as compared to controls, less conjugated metabolites in plasma and liver, decreased in vitro aniline hydroxylation and ethylmorphine N-demethylation. Finally, following acetaminophen administration, reduced hepatic glutathione was depleted to a much lesser extent in the tumor bearing animals than in controls. In conclusion, the L-1210 leukaemia seems to modify the acetaminophen hepatotoxicity and this effect might be explained by decreased acetaminophen biotransformation into toxic metabolites or intermediates.